1. Field of the Invention
The present invention relates to the field of combustion technology. It refers to a combustion device, particularly for driving gas turbines, comprising a plurality of burners of identical thermal power output, which work parallel to an axis into a common combustion chamber.
2. Description of the Related Art
Such a combustion device is known, for example, from the applicant""s EP-B1-0571782.
Thermoacoustic combustion instabilities may severely impede safe and reliable operation of modern gas turbines with premixing. One of the mechanisms responsible for these instabilities is based on a feedback loop which includes the pressure and velocity fluctuations in the fuel injection, the (convective) fuel inhomogeneities transported by the flow and the heat release rate.
A fundamental stability criterion for the occurrence of thermoacoustic combustion instabilities is the Rayleigh criterion which may be formulated as follows:
As soon as a flame is enclosed in an acoustic resonator, thermoacoustic self-starting oscillations may occur when the following applies                                           ∫            0            T                    ⁢                                    Q              xe2x80x2                        ⁢                          p              xe2x80x2                        ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                       greater than         0                            (        1        )            
Here, Qxe2x80x2 is the instantaneous deviation of the integral heat release rate from its mean (stationary) value, pxe2x80x2 designates the pressure fluctuations and T designates the period of the oscillations (1/T=f is the frequency of the oscillations). Formula (1) that the spatial extent of the heat release zone is sufficiently small to operate with integral values of Qxe2x80x2 and pxe2x80x2. An extension to the more general situation with a distributed heat release Qxe2x80x2 (x) and a small acoustic wavelength results directly and leads to a so-called Rayleigh index. The Rayleigh criterion (1) states that an instability can occur only when fluctuations in the heat release and the pressure are in phase with one another at least to a particular degree.
In a combustion device with premixing, the instantaneous heat release rate depends, inter alia, on the instantaneous fuel concentration in the premixed fuel/air mixture which enters the combustion zone. The fuel concentration, in turn, may be influenced by (acoustic) pressure and velocity fluctuations in the vicinity of the fuel injection device, presupposing that the air supply and the fuel injection device are not acoustically rigid. This lastmentioned condition is usually fulfilled, that is to say the pressure drop of the airflow along the fuel injection region of the burner is usually relatively slight, and even the pressure drop along the fuel injection device is generally not sufficient to uncouple the fuel feed line from the acoustics in the combustion device. The relation between the acoustics at the fuel injection device and the heat release in the flow may be formulated by means of the simplest possible explanations as follows:                                                         Q              xe2x80x2                        ⁡                          (              t              )                                Q                =                              -                                                            u                  xe2x80x2                                ⁡                                  (                                                            x                      1                                        ,                                          t                      -                      τ                                                        )                                                            u                ⁡                                  (                                      x                    1                                    )                                                              -                                    1              2                        ⁢                                                            p                  xe2x80x2                                ⁡                                  (                                                            x                      1                                        ,                                          t                      -                      τ                                                        )                                                            Δ                ⁢                                  xe2x80x83                                ⁢                p                                                                        (        2        )            
Here, x1 designates the location of fuel injection and u(x) and uxe2x80x2 (x) designate the flow velocity and its instantaneous change in time, while xcfx84 is the time delay which expresses the fact that fuel inhomogeneities occurring at the fuel injection device are not detected immediately by the flame, but only after they have been transported from the injection location to the flame front by the mean flow. In a self-igniting combustion device, xcfx84 is determined by the kinetics of the chemical reactions defining the location of the flame. By contrast, in a conventional combustion device with premixing, the flame is anchored by a flame holder which may assume different configurations (bluff body, V-gutter, recirculation zone or the like). The time delay depends in this case on the mean flow velocity and the distance between the injection location and the flame holder. At all events, the time delay may be described approximately by                               τ          =                                    ∫              0              l                        ⁢                          dx                              U                ⁡                                  (                  x                  )                                                                    ,                            (        3        )            
l designating the distance between the injection location and the flame front, while U(x) is the mean flow velocity in the premixing zone of the burner with which the fuel inhomogeneities are transported in the flow from the injection device to the flame.
It may be stated, in summary, that equation (2) expresses the fact that an instantaneous increase in the velocity of the air flowing past the fuel injection device (first term on the right side of the equation) leads to a dilution of the fuel/air mixture and a corresponding reduction in the heat release, while a pressure increase at the fuel injection device (second term on the right side of the equation) reduces the instantaneous fuel mass flow and therefore likewise lowers the heat release rate. It may be pointed out that, even when the fuel injection device is acoustically xe2x80x9crigidxe2x80x9d (that is to say xcex94pxe2x86x92∞), fuel inhomogeneities may be generated at the injection device.
As regards thermoacoustic stability, a time delay, such as occurs in equation (2), generally makes it possible to have a resonant feedback and an intensification of infinitesimal disturbances. The exact conditions and frequencies at which self-starting oscillations occur also depend, of course, on the mean flow conditions, specifically, in particular, the flow velocities and temperatures, and on the acoustics of the combustion device, such as, for example, the boundary conditions, natural frequencies, damping mechanisms, etc. The relation between the acoustic properties and the fluctuations in the heat release, such as is described in equation (2), is nonetheless a threat to the thermoacoustic stability of the combustion device which is to be taken seriously. A way should therefore be found to suppress this mechanism from the outset.
In principle, within the framework of the considerations referred to above, it is conceivable to bring about a suppression of thermoacoustic instabilities by a distribution of different time delays on the time axis. In this case, the injected fuel is divided into two or more individual streams or xe2x80x9cparcelsxe2x80x9d which all have time delays different from one another and correspondingly different phases. Ideally, such a division into various fuel streams would result in fluctuations in the heat release Qxe2x80x2i (i=1, 2, . . . ), such that                                           ∑            i                    ⁢                      xe2x80x83                    ⁢                                    ∫              0              T                        ⁢                                                            Q                  i                                ⁡                                  (                  t                  )                                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                    =        0                            (        4        )            
would be applicable. This would ensure that the Rayleigh criterion (1) cannot be fulfilled. In practice, such an exact cancellation is neither possible nor necessary; it is sufficient to lower the intensity of the resonant feedback to an extent such that the dissipative effects within the system are greater than the reinforcing mechanisms.
It was already proposed in the past (DE-A1-198 09 364), within a burner or in a plurality of burners working in parallel into a combustion chamber, to inject fuel in an axially graduated manner at different axial distances from the location of heat release. Part quantities of the fuel are thus transported with convective delay times of differing length from the location of injection to the flame, thus resulting in unequal phase relationships and therefore an attenuation of the resonant feedback. Such a solution has the disadvantage, however, that fuel injection is comparatively complicated in terms of apparatus because of the axial graduation: to be precise, if axially graduated injection takes place within a burner, it is necessary to have a plurality of separate injection orifices located one behind the other. If, on the other hand, a plurality of parallel burners are used with different axial injection locations, the burners have to be manufactured individually because of their different configuration, thus making production and stockkeeping considerably more expensive. At all events, in the case of fuel injection which is far upstream, there is also the increased risk of a so-called flame flashback which may lead to thermal overloading and destruction of the burner.
Other solutions, known from the prior art, to the problem of combustion instabilities relate to the distribution of the heat release along the axis of the combustion device by the generated flames or flame fronts being positioned so as to be distributed axially. U.S. Pat. No. 5,471,840 proposes, in this respect, to arrange at the individual burner, in each case on the outlet side, additional flame holders which displace part of the combustion (or flame fronts) upstream out of the combustion chamber into the flow pipe of the burner. A disadvantage of this, however, is that each burner has to be equipped with the flame holders. Another disadvantage is that the flame holders are subjected to high thermal load and therefore have to be cooled in a very complicated way and manufactured from material (ceramic) resistant to high temperature. Problems nonetheless arise with regard to long-term strength.
By contrast, it is proposed in U.S. Pat. No. 5,901,549 to use a pilot burner operating asymmetrically with respect to the axis, in order to generate longer and shorter flames at the adjacent premixing burners. One disadvantage of this is, above all, that a pilot burner operates in the diffusion mode, therefore generates high NOx emissions and consequently cannot be used in the full-load mode. Another disadvantage is that the pilot burner plays a central part in suppressing the combustion instabilities, so that disturbances in the pilot burner impair the functioning of the system as a whole. Furthermore, the necessary interaction between the pilot burner and the other burners is difficult to set and optimize.
The object of the invention, therefore, is to design a combustion device of the type initially mentioned in such a way that combustion instabilities are suppressed in a simple and functionally reliable way.
The essence of the invention is that the burners themselves are designed differently in such a way that the flames or flame fronts generated by them assume different axial positions and the heat release is thus distributed along the axis. The different design of the individual burners can be carried out without difficulty and with simple means and is feasible in the most diverse types of burners, without the need for complicated accessories. In particular, the parameters characteristic of the burner behavior may be selected differently from burner to burner, in order to obtain a corresponding axial flame distribution. An important advantage, in this case, is that all the burners used can be designed as premixing burners, so that this solution is compatible with a full load and entails virtually no disadvantages as regards NOx emission.
A preferred embodiment of the combustion device according to the invention is characterized in that the burners are designed as swirl-stabilized premixing burners, and in that the different axial position of the flames is brought about by a different swirl coefficient of the individual burners. Preferably, at the same time, the burners are designed as double-cone burners, into which the combustion air is injected in each case through slits formed between the cones; the different swirl coefficient is determined by the width of the slits and the aperture angle of the cones.
According to another preferred embodiment with swirl-stabilized premixing burners, the different axial position of the flames is brought about by the additional injection of air at the inlet and/or outlet of the burners. It is also possible, however, that, at the burners, the fuel is injected through injection orifices arranged in a distributed manner, and that the different axial position of the flames is brought about by a different arrangement and size of the injection orifices, or that the burners each have an outlet to the combustion chamber, and that the different axial position of the flames is brought about by a different configuration of the outlets.
Another preferred embodiment of the invention is distinguished in that the burners are designed as secondary burners, and in that a different axial position of the flames is produced in that the burners are equipped partially with a diffuser at their outlet to the combustion chamber and open into the combustion chamber partially without a diffuser.